Overweight and obese women may have a tougher battle in store when it comes to breast cancer: a new study published in the journal Cancer finds that carrying extra pounds is linked with a higher risk of cancer recurrence and death.

Previous studies have linked obesity with breast cancer recurrence, but the new study is among the first to find the same trend even among women who are overweight but not obese. The researchers found that having higher body mass index increased women’s risk of breast cancer recurrence and death, even if they had state-of-the-art treatment like chemotherapy and hormonal therapy.

“We found that obesity at diagnosis of breast cancer is associated with about a 30 percent higher risk of recurrence and a nearly 50 percent higher risk of death despite optimal treatment,” said lead study author Dr. Joseph Sparano of the Montefiore Einstein Center for Cancer Care, in New York City, in a statement.

For the study, researchers analyzed data on 6,885 patients with stage 1, 2 or 3 breast cancer who were enrolled in three National Cancer Institute–sponsored treatment trials. The researchers compared outcomes of obese and overweight women with those of normal-weight participants. Aside from their weight differences, all the women in the new study had normal heart, kidney, liver, and bone marrow function and were considered healthy overall.

Over eight years of follow-up, about 1 in 4 women saw their cancer come back and 891 died (including 695 women who died from breast cancer). The researchers found that the association between excess weight and cancer recurrence and death was strongest among women with estrogen receptor positive breast cancer, which is the most common type of breast cancer, affecting about two-thirds of all patients, according to the study authors.

The study wasn’t designed to identify the underlying factors connecting weight and breast cancer recurrence, but the hormone estrogen may play a role, the authors surmise. People with more fat stores produce more estrogen, which may fuel the growth of hormone receptor positive tumors.

Another theory is that heavier people are more likely to be insulin-resistant and therefore to have more insulin, another hormone that is thought to trigger the growth of breast cancer cells. Having excess body fat may also cause more inflammation in the body, which could drive breast cancer cells to spread or cancer to recur. “There are several possibilities and it could be any one of these factors or a combination of a few,” says Sparano.

The authors say obese women may do better with breast cancer treatment strategies aimed at such hormonal changes and inflammation. Perhaps these women need to be treated longer, or would benefit from lifestyle changes that would encourage weight loss — and improve health overall — after breast cancer diagnosis.

The authors call for more research into whether lifestyle modification could lead to positive long-term outcomes in obese breast cancer patients. “It’s possible that changes in diet could complement chemotherapy successfully,” says Sparano.